scripts: checkpatch.pl: warn on invalid credentials

@quicinc.com identities are not allowed.  Check the "From:" field in the
patch, equivalent to the author in the git commit, and the Signed-off-by
field.

Signed-off-by: Bryan Huntsman <bryanh@codeaurora.org>
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
[imaund@codeaurora.org: Resolved context conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>